Intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for a diseased vessel, such as an artery, within the human body to determine the need for treatment, to guide the intervention, and/or to assess its effectiveness. IVUS imaging uses ultrasound echoes to create an image of the vessel of interest. The ultrasound waves pass easily through most tissues and blood, but they are partially reflected from discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. The IVUS imaging system, which is connected to the IVUS catheter by way of a patient interface module (PIM), processes the received ultrasound echoes to produce a cross-sectional image of the vessel where the catheter is placed.
In a typical rotational IVUS catheter, a single ultrasound transducer element is located at the tip of a flexible driveshaft that spins inside a plastic sheath inserted into the vessel of interest. The transducer element is oriented such that the ultrasound beam propagates generally perpendicular to the axis of the catheter. A fluid-filled sheath protects the vessel tissue from the spinning transducer and driveshaft while permitting ultrasound signals to freely propagate from the transducer into the tissue and back. As the driveshaft rotates (typically at 30 revolutions per second), the transducer is periodically excited with a high voltage pulse to emit a short burst of ultrasound. The same transducer then listens for the returning echoes reflected from various tissue structures, and the IVUS imaging system assembles a two dimensional display of the vessel cross-section from a sequence of these pulse/acquisition cycles occurring during a single revolution of the transducer.
In the typical rotational IVUS catheter, the ultrasound transducer is a piezoelectric ceramic element with low electrical impedance capable of directly driving an electrical cable connecting the transducer to the imaging system hardware. In this case, a single pair of electrical leads (or coaxial cable) can be used to carry the transmit pulse from the system to the transducer and to carry the received echo signals from the transducer back to the imaging system by way of a patient interface module (“PIM”) where the echo signals can be assembled into an image. An important complication in this electrical interface is how to transport the electrical signal across a rotating mechanical junction. Since the catheter driveshaft and transducer are spinning (in order to scan a cross-section of the artery) and the imaging system hardware is stationary, there must be an electromechanical interface where the electrical signal traverses the rotating junction. In rotational IVUS imaging systems, this problem can be solved by a variety of different approaches, including the use of rotary transformers, slip rings, rotary capacitors, etc.
While existing IVUS catheters deliver useful diagnostic information, there is a need for enhanced image quality to provide more valuable insight into the vessel condition. For further improvement in image quality in rotational IVUS, it is desirable to use a transducer with broader bandwidth and to incorporate focusing into the transducer. A piezoelectric micro-machined ultrasound transducer (PMUT) fabricated using a polymer piezoelectric material offers greater than 100% bandwidth for optimum resolution in the radial direction, and a spherically-focused aperture for optimum azimuthal and elevation resolution. While this polymer PMUT technology offers many advantages, the electrical impedance of the PMUT is too high to efficiently drive the electrical cable connecting the transducer to the IVUS imaging system by way of the PIM. Furthermore, the transmit efficiency of polymer piezoelectric material is much lower compared to that of the traditional lead-zirconate-titanate (PZT) ceramic piezoelectric. Therefore, the signal-to-noise ratio of a PMUT will be compromised unless the deficiency in acoustic output can be compensated for by improved transmit electronics and/or other signal processing advances.
Current approaches to form a focused ultrasound beam include the use of an acoustic lens using conventional PZT transducers. For example, a rubber lens with an acoustic velocity of 1.0 mm/μsec has been used for elevation focus in phased array ultrasound systems. These approaches pose complex fabrication problems and the difficulty of removing imaging artifacts in the resulting signal.
Accordingly, there remains a need for improved devices, systems, and methods for implementing focused piezoelectric micro-machined ultrasonic transducers within an intravascular ultrasound system.